This invention relates to an apparatus for coating the surface of a continuous ribbon-like glass sheet with a metal oxide to alter some optical or electrical properties of the glass sheet by spraying a solution of at least one ogranometallic compound which can be thermally decomposed to the intended metal oxide onto the glass surface while the glass sheet is horizontally travelling in a heated state, the apparatus being of the type having a spray gun which is reciprocatingly movable above and transversely of the glass sheet. For example, the apparatus is useful in the production of heat-reflecting glass sheet.
It is popular to form a thin coating film of a metal oxide on the surface of a glass sheet for the purpose of altering certain optical or electric properties of the glass sheet. A prevailing method of coating the surface of a glass sheet with a metal oxide is spraying of a solution of at least one organometallic compound, which can be decomposed to the intended metal oxide by heating, in an organic solvent onto the surface of the glass sheet while the glass sheet is in a sufficiently heated state.
For example, this coating method is widely employed in the industrial production of heat-reflecting glass sheet and is often performed in the manner of a continuous process by using a spraying apparatus having a spray gun which is arranged above the path of a hot and horizontally travelling glass ribbon and is reciprocatingly movable across the width of the glass ribbon. The spraying apparatus includes exhaust ducts to discharge the decomposition gas evolved by the pyrolysis of the sprayed ogranometallic compound solution on the glass surface and the unreacted residual portion of the spray from the spraying zone.
As is well known, the optical properties and mechanical strength of the metal oxide coating film formed by this method depend significantly on the temperature of the gas atmosphere in the spraying zone and the efficiency of the exhausting of the decomposition gas and the residual portion of the sprayed solution. With due consideration of this matter, U.S. Pat. No. 4,064,832 shows a spraying apparatus including an outer enclosure and an inner hood which defines a spraying zone in its middle portion having two slant walls arranged symmetrically with respect to the path of the movement of a spray gun across the width of a glass ribbon travelling beneath and provides upright exhaust passages in its two end portions located adjacent and respectively upstream and downstream of the spraying zone. In this apparatus, the space between the outer enclosure and the inner hood is used as a gas passage to introduce a suitable quantity of an ambient gas heated by the heat of the hot glass ribbon into the spraying zone through a slot in the top wall of the middle portion of the inner hood. By using this spraying apparatus it is possible to form a metal oxide coating film such as a heat-reflecting film of fairly good optical and mechanical properties on the glass surface.
However, it has been recognized that a metal oxide coating film formed by using the spraying apparatus according to this U.S. patent is not always sufficient in the uniformity of its thickness and in the strength of adhesion to the glass surface. Probably, such a matter of unsatisfactoriness is attributed to some interfering influences of the flow of the introduced ambient gas in the spraying zone along the aforementioned slant walls on the flow of the sprayed solution toward the glass surface and the flow of the decomposition gas into the exhaust passages.